The United States Environmental Protection Agency (EPA) pursuant to the Clean Air Act requires gasoline to contain less than 1.0% benzene by volume beginning in the 1990's; this standard has been adopted by many countries throughout the world. To comply with this regulation, refineries have implemented various techniques to reduce the levels of benzene in gasoline, which otherwise contains approximately 2 to 3% benzene.
Gasoline is a well known fuel, generally composed of a mixture of numerous hydrocarbons including aromatics, olefins, naphthenes and paraffins having different boiling points at atmospheric pressure. The primary sources of the benzene in gasoline are the gasoline blending stocks which include naphtha from fluid catalytic cracker (FCC) units and catalytic reformer products (reformate). While FCC naphtha is the largest single blending stock for gasoline and constitutes up to 50% of the final product, the FCC naphtha itself typically contains only about 1% benzene and is therefore not the primary contributor of benzene. In contrast, reformate normally contains more than 5% benzene and given that approximately 75% of the benzene that is present in gasoline is derived from reformate, many strategies for reducing benzene levels in gasoline have focused on removing a substantial portion of the benzene from reformate prior to blending.
The most common techniques for reducing the benzene content in gasoline blending reformate include chemical processes that convert benzene to other desirable and less objectionable components for gasoline blending and physical separation that removes at least a portion of benzene.
An early approach was alkylation of benzene to yield heavier aromatics whose presence in gasoline was more acceptable. These techniques generally consisted of alkylating benzene with light olefins. Unfortunately, many of the alkylation processes were accompanied by undesirable side reactions and all of these techniques increased the costs to gasoline production significantly. Alkylation techniques are described, for example, in U.S. Pat. No. 3,293,315 to Nixon, U.S. Pat. No. 3,527,823 to Jones U.S. Pat. Nos. 4,140,622 and 4,209,383 both to Herout et al., and U.S. Pat. No. 4,849,569 to Smith.
Another feasible approach of reducing the levels of benzene in reformate was to convert benzene into cyclohexane. For examples, U.S. Pat. No. 5,773,670 to Gildert et al. discloses a process for the hydrogenation of aromatics in a petroleum stream, but the process is not selective only to benzene and therefore yields a number of undesired by-products. U.S. Pat. No. 6,187,980 to Gildert et al. discloses a process for hydrogenation of benzene to cyclohexane in a distillation column reactor where the feedstock is essentially pure benzene. U.S. Pat. No. 5,856,602 to Gildert et al. describes the hydrogenation of aromatics in a hydrocarbon stream in a distillation column reactor whereby the placement of the catalyst bed and operation of the distillation column determine which aromatics are retained in the catalyst bed for hydrogenation. U.S. Pat. No. 5,294,334 to Kaul et al. and U.S. Pat. No. 5,210,333 to Bellows et al. each disclose processes which selectively adsorb benzene from a gasoline stream and thereafter hydrogenate the benzene into cyclohexane without the need for added desorbents. A serious drawback of these approaches is that since the cyclohexane remains in the gasoline stream, there is a significant reduction in the grade of the gasoline because the octane rating of cyclohexane is much lower than that of benzene.
In order to partially recover the lost octane number, U.S. Pat. No. 5,830,345 to Lee et al. discloses a process for producing a benzene-reduced gasoline blending stock which uses a dual functional catalyst to hydrogenate benzene into cyclohexane and isomerize the cyclohexane into methylcyclopentane, which has an octane rating that is between that of cyclohexane and benzene. Again, this hydrogenation method (even with isomerization) adds to the refining costs and reduces the grade the gasoline blending stock.
Cyclohexane is commercially produced in huge quantities mainly through the hydrogenation of benzene with hydrogen gas in a fixed bed of nickel catalyst or a noble metal catalyst. Hydrogenation of aromatics is well known. For example, U.S. Pat. No. 2,373,501 to Peterson describes a liquid phase process for hydrogenating benzene to cyclohexane. U.S. Pat. No. 5,189,233 to Larkin et al. discloses a liquid phase process for benzene hydrogenation to cyclohexane whereby the liquid phase with the benzene is exposed to progressively more active catalysts. The process is said to be more selective to benzene and to provide higher yields. High pressures are employed to maintain the reactants in the liquid state.
U.S. Pat. No. 4,731,496 to Hu et al. discloses a gas phase process for hydrogenation of benzene to cyclohexane over a nickel catalyst supported on titanium oxide/zirconium oxide. In practice, benzene is contacted with hydrogen in the presence of the catalyst in a hydrogenation reactor operating at elevated temperatures and pressures. Good conversion of benzene to cyclohexane is achieved, although side reactions, such as cracking and isomerization, may occur. The cyclohexane product should have at least 99.9 wt % purity with less than 1,000 ppm of total impurities. The impurities include unconverted benzene, methyl cyclopentane (from isomerization), methyl cyclohexane (from the toluene impurity in benzene), and trace amounts of n-hexane, n-pentane, etc. (from cracking). Lowering the reaction temperature will minimize the production of by-products but at the expensive of added complexity and costs. For instance, a process that employs multi-stage reactors with inter-stage cooling to remove the heat from the highly exothermic hydrogenation reaction and higher recycling of unconverted benzene and cyclohexane product can achieve better benzene conversion. In particularly, for a four-stage reactor system, it was found that approximately 95% of the initial benzene was converted to cyclohexane in the first reaction vessel, while only 3.5%, 1%, and 0.3% of the initial benzene were converted to cyclohexane in the last three reaction vessels, respectively. Despite the better cyclohexane yield, the process is not economically attractive in view of the attendant complicated reactor system.
Examples of commercial operations that convert the benzene completely and yield high purity cyclohexane product include hydrogenation processes developed by UOP, BP and IFP. UOP's Hydrar process uses fixed platinum-based catalyst beds in a series of two or three reactors. The reaction temperatures are stepped up between 200 to 300° C. and the reactors operate at below 30 atm hydrogen partial pressure in order to achieve complete conversion per pass. To control the reactor temperature, a portion of the liquid product is recycled to dilute the benzene.
In the two-step BP hydrogenation process, the reaction temperature in stage one is controlled by recycling liquid and vapor from stage two and the effluent from the first stage is able to reach 95 wt % cyclohexane. Furthermore, temperature control of stage one by using both the sensible heat energy and heat of vaporization from the recycled cyclohexane affords good energy recovery with the added benefit that the recirculation rate of cyclohexane is within manageable limits. However, the advantages associated with the reduction in hydrogenation partial pressure and in temperature are offset by the fact that recycling means that a second reactor is needed complete the hydrogenation reaction.
Finally, the IFP process is carried out in liquid phase at 185° C. at a pressure of 20 to 35 atm and in the presence of nickel-based catalysts that are held in suspension by agitation with an external circuit. The cyclohexane product in the reactor effluent is in vapor phase thereby facilitating the partial removal of heat. A heat exchanger located in the external circuit removes the remaining portion of heat. Major drawbacks of this process are that the relatively large catalyst particles limit the catalyst activity and reduce the stability of the suspension. As is apparent, current commercial benzene hydrogenation processes which are designed to yield high purity cyclohexane products are overly complex.
The art is in need of a simple, economical method of removing purified benzene from feedstock such as reformate and converting the purified benzene into cyclohexane in a simplified hydrogenation reactor which is a part of an integrated process with interrelated steps.